Sensitizing photographic emulsions



Patented Nov. 5, 19 46 SEN SITIZING PHO TO GRAPHIC EMULSION S Samuel E. Sheppard, Rochester, N. Y., and Harlow It. Brigham, Oak Ridge, Tenn., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application July 13, 1944, Serial No. 544,812

4 Claims.

This invention relates to light-sensitive photographic materials and particularly to a method for sensitizing photographic emulsions.

Photographic emulsions which are sensitive to light by reason of the dispersion of silver halide in a colloidal suspension, such as gelatin may have their sensitivity enhanced in two ways. The spectral range to which the emulsion is sensitive may be extended as by th se of sensitizing dyes and this is usually spoken of as optical sensitizing. The sensitivity of the emulsion may also be increased for all spectral regions to which it is sensitive, and this is called chemical sensititizing. This enhancement, however, applies equally and independently to any "optical sensitize region of the spectrum. The present in vention relates to the latter type of sensitizing in which the sensitivity of the emulsion is increased in both its original spectral range and in any optically sensitized region, and the emulsion thereby given the property of recording an impression or image with less light energy than would be possible without the addition of the sensitizer or of giving a more dense image on processing or both.

It is known that the sensitizing effect of the chemical sensitizers is due to the formation of silver sulfide on silver halide under proper conditions. The silver sulfide must be formed in sufficiently finely divided state on the silver halide and this may be accomplished by using sulfur compounds as sensitizers which react with silver halide, and then decomposing to form the silver sulfide (Photographic Journal 1925, page 380). Among the sulfur compounds which have been used for this purpose are thiourea and various substituted thioureas. An objection to the use of the simple thiourea and substituted thioureas is that these compounds, being readily diifusible in the colloidal medium, tend to react or combine with the first silver halide grains which they meet so that some grains are overtreated and fog is produced, while other grains are starved. This results in an uneven sensitizing of the emulsion so that some portions of the emulsion are faster or more sensitive than others, and greater graininess and excessive fog arise before a useful enhancement of sensitivity is effected.

It is, accordingly, an object of the present invention to produce photographic emulsions having improved photographic properties because of the use of our sensitizing materials. A further object is to provide chemical sensitizers for emulsions which uniformly improve the light-sensitivity of the emulsion, that is, the ability to give the earliest developable impression or image with the least exposure, or the ability to give the maximum density after a given exposure, or both. A still further object is to provide non-diffusing sensitizers for photographic emulsions. A still further object is to provide a method for making photographic emulsions having definite controlled light-sensitivity properties throughout the spectral ranges to which they are normally or optically sensitive. Other objects will appear from the following description of our invention.

These objects are accomplished by adding to a gelatin emulsion with rapid and continuous stirring, or better to gelatin which is to be incorporated in an emulsion, a high molecular compound formed from a sulfur compound, such as thiourea or a substituted thiourea and an aliphatic acid or aliphatic alcohol containing six or more carbon atoms.

The compounds which we use as sensitizers are made by combining a thiourea having the following general formula:

in which two of the R's are hydrogen and the other two R's may be one hydrogen, one alkyl or an aryl group, with an aliphatic acid or aliphatic alcohol having one of the following formulae:

in which R1 represents an alkyl or aralkyl group of from 5 to 21 carbon atoms and R2 represents an alkyl group.

The compounds formed in this way have the following general formulae:

in which R1 represents an alkyl or aralkyl group containing from 5 to 21 carbon atoms and R3 represents hydrogen, an alkyl or an aryl group.

Among the thiourea compounds which may be used to form the sensitizers of our invention are thiourea, methyl thiourea, allyl thiourea, dimethyl thiourea, methyl phenyl thiourea and others. In these compounds there must be two hydrogen atoms attached to nitrogen, either two hydrogen attached to one nitrogen or one hydrogen attached to each of the nitrogen atoms.

The high molecular compounds which we use for condensation with the thiourea compounds include saturated and unsaturated alkyl and aralkyl aliphatic acids containing six or more carbon atoms, and aliphatic alcohols containing eight or more carbon atoms. The aliphatic acids which may be used include caproic, caprylic, lauric, myristic, palmitic, stearic, behenic, oleic, linoleic, gluconic, saccharic, undecylenic and hydroxystearic acids. The sulfonic acid derivatives of these acids may also be used. The acid components may be used in purified form or in mixed form as they are derived from cocoanut oil, palm oil, soya bean, linseed, olive oil, rape-seed, cottonseed oil, peanut oil, castor oil and the oils of animal origin, such as tallow, fish or seal oils or hydrogenated oils of these sources.

The high molecular alcohols which we may use include aliphatic alcohols containing an acid group or those containing a halogen atom linked to oxygen. Alcohols, such as octyl, dodecyl,

lauryl, octadecyl alcohols, geraniol, or phytol may a be converted into halogenated ethers and then treated with sodium thiosulfate to form the thiosulfuric acid derivatives. In addition to the thiosulfuric acid derivatives, alcohols may be used which contain SOaH or OH groups as substituents. It is necessary for the alcohol to contain an acid or OH group in addition to the hydroxyl group of the alcoholic CHzOH group.

A typical reaction in the formation of higher molecular sensitizers, according to our invention, may be represented by the following equation for the condensation of palmitic acid with thiourea:

NHz

CHa(CH2)14C 0 01+ In the formation of the high molecular com pounds from palmitic acid and thiourea, one mol. of chloride of the palmitic acid was dropped into a solution containing 1.2 mols. of thiourea at a temperature of just under 10 C. and the mixture agitated vigorously. A reaction took place resulting in the compound consisting of rather greasy crystals which were recrystallized from alcohol.

The compounds used as sensitizers according to our invention are found to greatly increase the light sensitivity of photographic emulsions when present in very small amounts. The high molecular sensitizers are prepared in concentrated and purified form so that when they are incorporated in gelatin or photographic emulsions they produce the desired increase of sensitivity without diluting the gelatin or emulsion to any great extent or impairing the physical properties of the emulsion. The sensitizers, therefore, constitute a dependable and easily controlled means for in- 4 creasing the sensitivity of photographic emulsions. The minute amounts added were found to enhance uniformly the sensitivity of the emulsion and did not impair its physical or chemical characteristics.

Another compound which we have found useful as a sensitizer according to our invention is styrene fumaryl allyl thiourea. This compound was prepared from the copolymer of styrene and fumaryl chloride, made as described in Voss et al. U. S. Patent 2,047,398. Five grams of the styrene fuxnaryl chloride polymer in acetone were added to 46 grams of allyl thiourea in acetone. The white precipitate was dissolved in ethylene chlorohydrin and reprecipit'ated by pouring into acetone.

In order to incorporate the sensitizing compounds in gelatin or in an emulsion, they are dissolved in a solvent which is photographically inert and this solution then added to the gelatin or emulsion. The resulting gelatin solution may be used immediately or may be dried and used later in the emulsion-making process. However, the sensitizers may be incorporated in an emulsion at any stage in its preparation subsequent to precipitation of the silver halides, preferably at the last stage just prior to the final digestion.

Operative concentrations of the sensitizers used according to our invention were found to vary from .0001% to over 001% based on the weight of dry gelatin in the emulsion. Thi range is not to be taken as limiting since it is specified only by way of example, it being possible to use higher or lower concentrations of sensitizer in the emulsion. However, it is undesirable to use very high concentrations of sensitizer as this is likely to produce fog in the resulting emulsion. We have found that the following concentrations of typical compounds are satisfactory:

Per cent .0001 to .0008 .0002 to .0006 .0001 to .0016

Our sensitizing materials may be used for increasing the general sensitivity of ordinary known color-sensitized emulsions as well as orthochromatic, panchromatic, X-ray and other special emulsions. They can be used with sensitizing dyes which increase the spectral range of sensitivity of the emulsion.

Although we have referred throughout the present specification to the use of gelatin and gelatin emulsions, it is to be understood that other colloidal materials may be used as the carrying medium for sensitive salts and sensitizing compounds of our invention. Other organic colloids and mixtures of them with each other or with gelatin where such colloids are suitable for making lightsensitive emulsions, can be used. Albumens, gums, celluloslc derivatives, synthetic resins and other compounds can be used as the carrying medium for the emulsion in place of gelatin. It is also to be understood that where we have referred to light-sensitive salts we intend to include silver chloride, silver bromide, and silver iodide either alone or in combination as the lightsensitive salt to which our sensitizing materials are added. It is also to be understood that other modifications may be made in our process or materials without departing from the invention and they are intended to be taken as limited only by the scope of the appended claims.

What we claim is:

l. A photographic emulsion comprising a colloidal medium containing light-sensitive silver Palmityl thiourea Palmitylallyl thiourea Styrene fumaryl allyl thiourea in which R1 is selected from the class of alkyl and aralkyl groups containing 5 to 21 carbon atoms and R3 is selected from the class of hydrogen, alkyl, and aryl groups.

2. A photographic emulsion comprising gelatin containing light-sensitive silver salts, and having added thereto a sensitizing compound of the general formula H RI 6 in which R1 is selected from the class of allwl and aralkyl groups containing 5 to 21 carbon atoms and'R: is selected from the class of hydrogen,

alkyl, and aryl groups.

3. A photographic emulsion comprising gelatin containing light-sensitive salts, and having added thereto a sensitizing compound of the general formula Rr-C ONH m-Nn in which R1 is selected from the class of alkyl and aralkyl groups containing from 5 to 21 carbon atoms and R3 is selected from the class of hydrogen, alkyl and aryl groups.

4. A photographic emulsion comprising gelatin containing lightsensitive silver salts, and having added thereto from 0.0001% to 0.001%, based on the weight of gelatin in said emulsion, of palmityl thiourea as a sensitizing compound.

SAMUEL E. SHEPPARD. HARLOW R. BRIGHAM. 

